The present invention relates to a piston for the variable delimitation of a pressure chamber in a housing of a hydraulic cylinder. In particular, the invention relates to a piston for master cylinders of hydraulic clutch actuating or brake systems in motor vehicles, as used extensively in the automotive industry.
These pistons are used to generate a pressure in the pressure chamber with an axial relative displacement in relation to the housing of the hydraulic cylinder. This pressure is possibly applied to a clutch slave cylinder hydraulically connected to the hydraulic cylinder, which is actively connected to the clutch to disengage a clutch. The piston considered here, also known as a plunger piston because of its design, has a main part with a running surface for (at least) one sealing element and an after-running device. The sealing element is attached to the housing of the hydraulic cylinder and serves to seal the pressure chamber in an operating position of the piston, i.e. with a piston displaced in the direction of the pressure chamber, in co-operation with the running surface. In a normal position, i.e. a position of the piston drawn to a stop, the after-running device connects the pressure chamber to an after-running area which, in turn, is connected to an after-running tank.
Prior art does not lack proposals on how the after-running device should be designed. For example, a piston is known which consists entirely of plastic for economic reasons (DE 38 16 608 A1), whose end on the pressure chamber side is provided with slots running in the longitudinal direction, which form the after-running device in a simple manner. However, this type has the disadvantage that with a relative displacement between plastic piston and sealing element, a noise is generated, which is undesirable in the automotive industry, which appears to be caused by the surface structure of the plastic.
Therefore, it has been suggested that to make the piston, a plastic body is covered with a thin-walled metal tube, at least in the area of the running surface (DE 37 13 248 C2) or is provided with a piston shank sleeve shaped from metal (see the generic DE 195 23 215 A1 for example), which has a closed floor on the pressure chamber side. In the case of these pistons, the after-running device is formed by sniffer grooves extending in the axial direction, which are incorporated in the surface of the piston shank sleeve on the end of the piston shank sleeve facing the pressure chamber. This is normally done without machining, i.e. using an embossing process.
An embossing process of this type does represent an economic production method, but it is also associated with disadvantages. For example, a sharp, dimensionally precise outlet of the sniffer grooves distributed on the perimeter cannot be guaranteed. Consequently, the grooves may have a different length. So that this does not affect the function of the after-running device when the piston is in its normal position, the sealing element has to be positioned in the housing of the hydraulic cylinder with very big tolerances. However, this means that the piston has to travel longer distances before pressure can be built up in the pressure chamber, which ultimately leads to an undesirable loss of pedal stroke. There is also the fact that as the result of the embossing process, an anti-corrosion surface coating applied to the piston shank sleeve may be damaged and detached, which leads to undesirable leakages in operation sooner or later. The same applies to metal-coated surfaces of plunger pistons otherwise made 100% in plastic.
Finally, pistons are known which are made from a solid material, like an aluminum alloy, the equalisation grooves provided on the end on the pressure chamber side being made by groove milling cutters. However, the equalisation grooves made in this way require considerable deburring to prevent damage to the sealing element in operation. As in the case of the piston designs described above, there is also the risk that the running surface of the piston is damaged if the piston has to be held or clamped to make the equalisation grooves.
The object of the invention is to create a piston for hydraulic cylinders that is easy to make and which, with reference to the after-running device, has an improved functionality compared with the prior art described.
According to the present invention, there is provided a piston for the variable delimitation of a pressure chamber in a housing of a hydraulic cylinder, in particular of a clutch master cylinder for motor vehicles, the piston having an operating position and including a main part, on which a running surface is provided for a sealing element on a housing side, which, in the operating position of the piston, seals the pressure chamber, and the piston further including an after-running device which, in a normal position of the piston, connects the pressure chamber to an after-running area; wherein the after-running device is made separately from the main part and is connected to the main part without play to form the piston.
Through this two-part design of the piston, the after-running device can be made with small tolerances in a simple manner without the risk of damaging the running surface provided on the main part of the piston and having to be after-worked before the after-running device is connected to the main part of the piston and therefore without creating further dimensional differences. As a result, the sealing element can be positioned in the housing of the hydraulic cylinder advantageously with smaller tolerances and the hydraulic cylinder is thereby improved with reference to pedal stroke losses. A further advantage of the two-part piston design is that it allows economic modular solutions. For example, it is possible to use the same after-running device on main parts of different lengths in order to make pistons which allow a stroke corresponding to the requirements concerned.
In one advantageously simple embodiment of the piston, the after-running device can be made as an annular part with an essentially U-shaped cross-section. The play-free connection between the after-running device and the main part is preferably made using a clip connection, which allows an easy assembly of the piston.
If the outside diameter of the running surface is slightly bigger than the outside diameter of the after-running device, and a sloping transition section is provided on the main part between the running surface and the after-running device, the sealing lip of the sealing element in the normal position of the piston is advantageously relieved slightly in contact with the after-running device, whereas with a movement of the piston from the normal position to an operating position via the transition section, the sealing lip is carefully expanded.
In a preferred embodiment of the piston, the after-running device has a radially outer annular section and a radially inner annular section, which are connected to each other via an annular disc section on the end. In this case, the outer annular section of the after-running device can have a cylindrical outside perimeter surface on which the sealing element rests in the normal position of the piston and which is provided with several equalisation grooves distributed over the perimeter, which extend from the free end of the outer annular section in the axial direction in order to ensure, in the normal position of the piston, the connection between the pressure chamber and the after-running area under the sealing element or its sealing lip. Preferably, the equalisation grooves on the outside perimeter surface of the outer annular section extend over the entire length of the outside perimeter surface, which allows easy manufacture, among other things.
In addition, the outer annular section of the outer running device can have a cylindrical inside perimeter surface, by means of which the after-running device radially centers in an advantageously simple manner on a centering shoulder of the main part and which is provided with several equalisation grooves distributed over the perimeter, which extend from the free end of the outer annular section in the axial direction and whose axial length is greater than the width of the centering shoulder in order to allow a hydraulic connection over the centering shoulder. In addition, the free end of the outer annular section of the after-running device forms an annular shoulder, as described in patent claim 10, with which the after-running device is supported without play on the main section in the axial direction in a simple manner and which is provided with several connecting grooves which run in the radial direction. The connecting grooves on the annular shoulder can connect the equalisation grooves on the outside perimeter surface to the equalisation grooves on the inside perimeter surface of the outer annual section. The annular disc section of the after-running device can be provided with at least one connecting duct extending in the axial direction.
It is evident that according to the embodiment of the after-running device described above, a connection between the pressure chamber and the after-running area can not only be achieved via an outer area of the after-running device if the piston is in the normal position, but also advantageously via an inner area of the pressure chamber via the connecting duct in the annular disc section, the equalisation grooves on the inside perimeter surface of the outer annular section and the connecting grooves on the annular shoulder of the outer annular section. Through this embodiment of the after-running device, the undesirable xe2x80x9cresidual pressure buildxe2x80x9d can be avoided in the pressure chamber in a simple and reliable manner.
If the position of the sealing element as already discussed above is improved with reference to pedal stroke losses, i.e. the stroke, also referred to as the over-running or valve closing stroke, which the piston has to cover starting from the normal position until the sealing element with its sealing lip is released from the after-running device to separate the pressure chamber and the after-running area, is minimised, there is basically the risk of a xe2x80x9cresidual pressure buildxe2x80x9d in the pressure chamber. This then means that the sealing lip of the sealing element pressed against the running surface of the main section in an operating position of the piston because of the pressure in the pressure chamber, particularly at high temperatures of the hydraulic fluid, may be pressed against the counter surface in the normal position of the piston, too, and therefore (partly) closes the passage to the after-running area with a minimised over-running stroke. As a result, a residual pressure in the pressure chamber is not reduced, or only with a time lag, in the direction of the after-running area after the piston returns to the normal position. In the case of hydraulic clutch operations, this can mean that the clutch abrades with excessive wear of the clutch lining or only a reduced torque is transmitted.
The connection described above between the pressure chamber and the after-running area via the inner area of the after-running device also provides a remedy here in a simple manner. Depending on the design and position of the sealing lip of the sealing element, and as a function of the residual pressure in the pressure chamber, a pressure equalisation is created via this inner connection in relation to the area of the compressed sealing lip facing away from the pressure chamber, thereby creating a partial pressure equalisation at the sealing lip or lifting the pressed sealing lip away from the counter surface, so that a pressure equalisation in relation to the after-running area is allowed as a result. In other words, through the inner connection created with the after-running device, in the normal position of the piston a hydraulic pre-tension of the sealing lip at the sealing element can be reliably eliminated or avoided.
The inner annular section of the after-running device can be slotted several times to form spring tabs, each of the spring tabs having a lug on the end projecting radially inwards, which can be engaged with a radial groove made in a fixing shoulder of the main section. Therefore, the above clip connection is created in a simple manner. Preferably, in this case, the lug is provided with a sloping surface on its side facing the pressure chamber, which excludes an otherwise undesirable axial play that might exist.
The after-running device can be made in an advantageous way as a one-piece plastic injection molding, which is not only cost-effective, but also guarantees the functionally desirable small tolerances in a simple manner and without the need for reworking.
Finally, the main part can be a solid body of preferably an NF metal to which a tubular sleeve forming the running surface or a coating, is applied. However, it is also possible for the main part to be an essentially pot-shaped body preferably made from sheet steel, which, if appropriate, surrounds a lining, preferably in plastic. As a result, the known running surface designs, optimised from the point of view of noise behaviour and economics, can be maintained.